Tools of this kind are intended for portable use without being connected to an electric power system. They are guided by a person operating them and as a rule are also carried while in use. For this reason, it has always been a goal to provide tools of this kind that are as small, handy and lightweight as possible, so as to facilitate handling and especially carrying of the tool. Known tools of this type, having an externally ignited internal combustion engine, are accordingly often equipped with magnetic ignition.
In a known tool of this kind, the motor-driven chain saw Type 028 made by Andreas Stihl, a permanent magnet is disposed at the outer periphery of the fan wheel and travels past and at a slight distance apart from an iron yoke having a coil mounted thereon. The fan wheel is mounted on the crankshaft of the engine, so when the motor is running, a voltage is induced in the coil by means of the travel of the magnet past the iron yoke, and this voltage, stepped up by a transformer, serves to generate the igniting spark.
A disadvantage of the known configuration is that the fan wheel has to be relatively stable and hence heavy to hold the magnet mounted on its outer periphery. In order to avoid imbalance of the fan wheel, a counterweight must also be provided on the fan wheel, offset from the magnet by 180.degree.. Since the centrifugal forces of these two weights increase quadratically with increasing rpm, they must be tightly anchored on the fan wheel and the latter itself must be configured to be correspondingly stable. As a result, first the fan wheel is relatively heavy, which increases the overall weight of the tool. Second, the fan wheel, revolving at a high rpm, generates strong centrifugal forces during operation, which makes handling the tool and in particular rotating it about its longitudinal axis difficult. Furthermore, the disposition of the two weights (magnet and counterweight) at least partly impedes the fan wheel in its function, because no fan blades, or only fan blades having limited functional effect are located at the region of the magnet and counterweight and this reduces the cooling air flow.